engleski

The decline of native molluscs on the Israeli shallow shelf

The Levantine basin in the eastern-most Mediterranean Sea hosts hundreds of non- indigenous species introduced after the opening of the Suez Canal. An equally dramatic phenomenon is the disappearance of native molluscs. Here we quan-tify this decline on the IsraeliMediterranean soft-substrate shelf based on samplingalong two transects off northern and southern Israel in Autumn 2016 and Spring 2017 at 10-40 mdepth. We compared the living assemblages with 1) a comprehensive literature- based checklist of Israelimolluscs filtered by appropriate substrate and depth, and 2) the compo-sition of the death-assemblagecollected with the living organisms. We used a smallmesh size (0.5 mm) to retain small sized and juvenile individuals and deployed an intense identificationeffort, including tracing the morphology of early post-larval stages. Our sampling intercepted only 24% of the historically recorded species, notwithstanding a sample coverage of 99.4%. At individual sites, the living assemblage native richness is between 2.9%and 18.5% of the death assemblage native richness.By contrast, we intercepted 54% of the his-torically recorded non-indigenous species. At individualsites, the living assemblage non- indigenous richnessranges between 7.1% and 80.2% (mean 42%) of the death assem-blage, with lower values in deeperstations. The abundance of native species peaks in spring (80%, 934 individuals) but drops in autumn to only 15% (279 individuals, not-withstanding that two additional replicates were collected) suggesting a mass mortality during summer. The comparison of size-frequency distributions from living and death assemblages showed that most native species are represented by small living individuals that do not reach the large sizes of the past, suggesting that while recruit-ment does occur, death occurs prematurely, for some species possibly before maturity. By contrast, for most non- indigenousspecies the size range of living individuals overlapswith the dead ones. These two results combinedsuggest that current environmental conditions(likely including the locally rapid climate warming) disproportionally favour non-indigenous species andthat native species reproduction may be hindered, with recruitment potentially coming from deeperwaters or other sectors of the basin.

molluscs, shelf, Mediterranean Sea, Israel

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